A variety of traffic safety facilities such as traffic signs, pavement markers, delineators and tripods or goods which should be importantly confirmed their visibility at rainy day or at night such as safety clothes, automobiles, bicycles, helmets and shoes are installed with or attached with a retroreflective article enabling to increase the visibility for the goods by retro-reflecting the incident light from front side thereof toward the light source radiating the light.
Conventionally, a retroreflective article provided with a glass bead or a cube corner has been usually used as the retroreflective article which will be applied to the goods which should be importantly confirmed their visibility.
However, the conventional retroreflective article has an inferior retro-reflection range at high angles of incidence because for the light having high incident angle the retroreflective article has low retroreflective ratio which is expressed as the ratio of the quantity of retro-reflected light to the quantity of incident light is low.
For example, in the case of the conventional retroreflective article using glass beads, because the light incident on the edges of the glass beads or into gaps between the glass beads is not well retro-reflected, the overall retro-reflection ratio is lowered, and thus brightness is low.
In the case of the conventional retroreflective article using cube corners, the overall retro-reflection ratio is high compared to the conventional retroreflective article using glass beads. However, when the incident angle of the light becomes large due to movement of the light source, the apparent area of the exposure surface (i.e. the area of the exposure surface when viewed from the direction of the light source) has no alternative but to be geometrically reduced. At this time, because the percentage of the retro-reflection area capable of retro-reflecting the light incident on the exposure surface is further reduced, the conventional retroreflective article using cube corners has a problem in that the brightness is sharply lowered in proportion to the magnitude of the incident angle. Therefore, the retro-reflection range of the incident angle, i.e. the visible retro-reflection range, is very narrow, and a retroreflective article enabling to retroreflect light having a large incident angle, which is deflected off the front of the exposure surface in a specific direction at an angle greater than a predetermined angle, is very difficult to design and fabricate.
To improve the problems of the conventional retroreflective article like above, retroreflective articles such as those of U.S. Pat. No. 4,349,598, entitled “High Incidence Angle Retroreflective Material” or U.S. Pat. No. 4,895,428, entitled “High Efficiency Retroreflective Material” have been developed. These retroreflective articles can retroreflect mainly the incident light having its incident angle greater than about 45 degree in which its main reflecting direction is sharply deflected to a specific direction.
These retroreflective materials consist of reflective elements formed with a cube corner between two rectangular surfaces which meets the dihedral angle of about 90 degree and one triangle surface which perpendicularly crosses the two rectangular surfaces are arranged, and they have very high retroreflective ratio for the incident light having high incident angle since they are sharply deflected in a specific direction which is a main reflective direction having the highest retroreflective ratio.
Further, “Highly canted retroreflective cube corner article” disclosed in International publication No. WO 1998/20374 is a varied cube corner structure, and this also has very high retroreflective ratio for a specific incident light having high incident angle.
However, the above mentioned retroreflective materials retroreflect using three reflecting surfaces as same the cube corner which is a conventional retroreflective structure, and therefore they still have a problem which is raised in conventional retroreflective articles using cube corners, that is, the problem which they have high retroreflective ratio only for a specific incident light having incident angle greater than 45 degree but it has very low retroreflective ratio for a incident light having low incident angle, therefore the incident range enabling to retroreflect is very narrow. Further, the above mentioned retroreflective materials is not freely designed in its main reflective direction defined as the highest retroreflective ratio.